The invention relates generally to direct-conversion radio receivers and to methods in direct conversion, especially for phase modulated carrier signals transmitting digital information. The invention also relates to suppression of direct current (DC) offsets generated in receivers.
In digital modulation, the modulating signal is digital, containing a bit stream of “1s” and “0s”, and mixed in a modulator with a carrier signal for transmission. The basic modulation schemes are amplitude-shift keying (ASK), frequency-shift keying (FSK) and phase-shift keying (PSK). In PSK, the signals representing the states “1” and “0” have a difference in phase. In binary phase-shift keying (BPSK), the phase has two different states. In quadriphase-shift keying (QPSK) modulation, the phase has four different states. In digital modulation changes in the modulated signal waveform spread the power of the signal to a wide frequency range. The spectrum of the signal comprises usually a wide main lobe at a carrier frequency. The main lobe has smaller side lobes around it on a frequency scale. Before modulation, at a direct current (DC) or baseband, the spectrum is just one half of this and the carrier frequency represents a zero frequency (0 Hz). The lobes are separated by notches, i.e. null points situated at multiples of the data rate (including the multiple of 1), i.e. bit rate (bits per second), from the carrier frequency.
Direct-conversion receivers, i.e. zero-IF receivers are known in the art. In a zero-IF receiver (IF, Intermediate frequency) received signals are mixed with an output of a down conversion oscillator to translate the received signal to the baseband. A down conversion oscillator is a local oscillator generating a signal on the carrier frequency. The phase-modulated input signal is split into two branches and the frequency of the local oscillator is mixed with the two branches (the other one with a 90° difference in phase). The output mixed signal in a branch without any phase shift is the in-phase signal (I), and the other one having a 90° difference in phase is the quadrature signal (Q). Lowpass or complex filters are provided to remove undesired sum products caused by mixer nonlinearities from the mixing. I/Q signals are preamplified, if needed, and input to analog to digital converters (ADCs) for signal processing. A demodulated signal is calculated from the I and Q signals using a processor system (DSP, Digital signal processing). The I and Q signals also exhibit DC offsets.
In direct-conversion receivers the pure carrier signal gives rise to a DC signal at the mixer output. Other undesired signals at the mixer input give rise to mixing products, the spectrum of which is located around the DC. Other sources of DC offsets include the synchronization of a local oscillator at a carrier frequency and the DC offsets in amplifiers and other circuit elements of the receiver due to temperature, aging, crosstalk, etc. The high gain of the baseband circuitry of the receiver amplifies DC offsets to the extent that the operating range of the circuitry is exceeded. Often the DC offset is compensated using a DC nulling circuitry that measures the offset before the reception and cancels it by means of a voltage that is charged into a large capacitor. High-level transmitters, interfering the reception of a signal of a lower level transmission signal, and starting or stopping their transmission during reception of a spread spectrum device change the DC offset and thus cause disfunctionality in the receiver. The DC offset compensation should be active during the actual spread-spectrum reception, which, on the other hand, results in a more complicated compensation circuitry.
DC offset voltages have a large dynamic range when compared to a useful signal spectrum, resulting in amplifier saturation or problems with the ADC conversion. One method to block the DC offsets is to AC couple the output of the mixer with a large coupling capacitor for generating a narrow notch at a DC frequency. Large capacitors are used to facilitate the use of very low corner frequencies near the zero frequency. The portion of the modulated signal centered around the carrier frequency is also lost, wherein distortion is caused in the demodulation, since the DC notch frequencies contain information. Large capacitors require high current drive capability consuming a lot of power and using a lot of space on printed circuit boards (PCBs).